Nanocrystal quantum dots show high photoluminescence quantum yields and size-dependent emission colors that are tunable through the quantum-confinement effect. Despite these favorable light-emitting properties, nanocrystals are difficult to use in optical amplification and lasing. Because of almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur due to nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by nonradiative Auger recombination, a process in which one exciton recombines by transferring energy to another.
Numerous technologies including optical interconnects in microelectronics, lab-on-a-chip, chemo- and bio-analyses, optical telecommunications, and information processing would benefit greatly from flexible, chemically processable optical-gain materials that can be manipulated using simple solution-based techniques. One class of materials is colloidal semiconductor nanocrystals (NCs) also known as NC quantum dots. NCs are nanoscale crystalline particles surrounded by a layer of organic ligand molecules. The dual inorganic-organic nature of these structures provides great flexibility for controlling their physical and chemical properties. For example, using quantum-confinement effect, the NC emission energy can be tuned by hundreds of milli-electron volts by simply changing the inorganic-core size. On the other hand, relatively straightforward surface chemistry can be applied to tune NC chemical reactivity to facilitate their incorporation into, e.g., nanophotonic or nanoplasmonic feedback structures for fabricating micro-lasers of various configurations.
Well-passivated NCs are characterized by near-unity photoluminescence (PL) quantum yields. Despite these favorable light-emitting properties, NCs are difficult to use in lasing applications. Because of the degeneracy of the lowest-energy emitting levels, population inversion in NCs can only be achieved if the average number of electron-hole pairs (excitons) per NC, <N>, is greater than 1, which implies that at least some of the NCs in the sample must contain multiexcitons. A significant complication arising from this multiexcitonic nature of optical amplification in NCs is highly efficient nonradiative Auger recombination induced by confinement-enhanced exciton-exciton (X-X) interactions. This process results in fast optical decay characterized by picosecond time scales. Demonstrated approaches to reducing Auger rates include the use of elongated NCs or quantum rods (see, Kazes et al., Adv. Mater. 14, 317-321 (2002) and Htoon et al., Phys. Rev. Lett. 91, 227401-1-4 (2003)) or core-shell hetero-NCs that allow the decrease of X-X coupling without losing the benefits of strong quantum confinement (see, Ivanov et al., J. Phys. Chem. B 108, 10625-10630 (2004)).
However, the most radical strategy for solving the problem of Auger decay is the development of methods or structures that would allow realization of optical gain in the single-exciton regime, for which Auger recombination is simply inactive.